Temperature compensated resonant cavity



J1me 1952 A. D. EHRENFRIED ,225

TEMPERATURE COMPENSATED RESONANT CAVITY Filed March 29, 1946 l/VVEN TOR.

ALBERT o. EHRENFRIED M40. 9, M. ATTORNEY Patented June 10, 1952 UNITE-JD smrss PATENT oFri-cE Albert D. Ehrenfried, BostonQMass, assignor, by mesne assignments, to United States of America as represented by the Secretary of War Application MaP'ch 29, 1946, Serial No. 657,929

This invention relates to resonant cavities, and particularly to means for compensating for the effects of thermal expansion on frequency.

In high frequency cavity resonators, the frequency of oscillation thereof is critically dependent upon the physical dimensions of the cavity, and undesirable detuning often accompanies small changes in ambient temperature. For-precision R.-F. work, it is therefore necessary to eliminate as completely as possible changes in cavity dimensions with temperature.

It is not always possible to use alloys having zero coefiicients of expansion because of cost, difficulty in production, or improper electrical characteristics.

The present invention provides a means of eliminating the undesired temperature expan sions and contractions of a cavity resonator which may be made of any electrically desirable metal regardless of its thermal expansion proper= ties.

In addition, temperature compensation apparatus of the present invention affords a simple means of tuning a cavity resonator without the use of a conventional tuning plunger which introduces mode limitations and adds mechanical complexity to existing structures.

In the drawings:

Fig. 1 is a fragmentary sectional view of a resonant cavity'embody-ing the invention.

Fig. 2 is a sectional view of a portion of the wall of the cavity.

The cavity to which the compensating structure of the present invention is applied is indicated in Fig. 1 as a cylinder 1, havingplanar end members 2 and 4 separated by a critical'distance L. The resonator shown sufiers both longitudinal and radial changes in dimensions with changes in temperature, and compensation for such dimensional variations is provided by temperature control of the length L.

The cylindrical wall I of the cavity is so formed as to have an annular portion -5 extending radially outward therefrom. Annular portion 5 may be substantially rectangular in cross-section, as shown, with its longer dimension N normal to the axis of the cylinder 1, and its shorter dimension P parallel thereto.

Two fulcrum rings 6 and l are disposed coaxially around cylinder l with knife edge portions 9 and I9 respectively thereof engaging the opposed sides II and I2 of the annular portion '5. Edges 9 and 10 thus form fulcrums with which levers formed by walls H and i2 of the annular portion 5 cooperate. Rings 6 and I are disposed 7 Claims. (Cl. 17844) within annular ring mountings M and I5 respectively, which are joined adjustably as by cooperating inner and outer threaded portions I6 and H.

In apparatus as shown in the accompanying figure, the thermal coefficient of expansion of the material of mounting rings I4 and I5 is chosen substantially lower than that of the cylindrical cavity wall I. Fulcrum edges 9 and!!! thus remain at a substantially fixed distance apart despite changes in temperature.

Dimensions of the cylindrical cavity wall I do, however, change with temperature and in such a manner that the length L of the cavity shown tends to increase with rise in temperature. It will be noted that the cylindrical wall section of the annular portion 5 having length P also tends to increase with rise in temperature. With radially extending walls I I and I2 acting as levers cooperative with substantially fixed fulcrum edges 9 and In, an expansion of wall length P causes a reduction in the length G of the gap between inner ends l9 and 20 of the cavity wall. By proper positioning of fulcrum edges 9 and -l 0, the expansion of a given wall length -P can be multiplied and made to lessen gap dimension G by an amount equal to the undesired increase in cavity length L. Complete or even over-compensation of linear temperature expansions can thus be achieved by apparatus of this invention.

In some cases, it may be desirable to stiffen the opposed sides H and [2 so that the lever action may be fully effective, and the wall thickness at the inner ends I 9 and 20 may be reduced to permit easier bending of the junctures between the annular portion and the cylindrical wall I. A sectional view of a portion of the wall of reduced thickness is shown in Fig. 2.

In order to prevent the cavity formed by annular portion 5 from interfering with the proper mode of oscillation of the resonator itself, the dimension N of the space normal to the axis of the cavity is selected to be one-half wavelength. The annular space thus acts as a choke to prevent oscillations from occurring therein, in accordance with principles well known in the art.

Not only does the lever and fulcrum arrangement provide for automatic compensation for thermal expansion but it can be used for frequency adjustment as well. Ring member 14 may be rotated at will to vary the spacing between the fulcrum edges 9 and I 0. It is desirable to permit the fulcrum edge to remain stationary when the adjusting ring I4 is rotated, to prevent wearing away or cutting of the annular member I i. Hence anti-friction means such as a ballbearing ring 2| may be installed between the rin member [4 and the fulcrum ring 6. Rotation of ring 14 will produce a linear change in the spacing between the knife edges 9 and i8, and hence of cavity dimension L.

It will thus be apparent that the form of the invention shown affords a simple and practical means of compensating for the undesired thermal expansions and contractions of a cavity resonator and for providing a desirable form of tuning adjustment. The scope of this invention is not limited to the particular embodiment shown and described above. Apparatus employing the principle herein set forth may be employed to compensate for any type of linear temperature expansion or contraction.

What is claimed is:

l. The combination, with a walled resonant cavity, of means for compensating for the effects of thermal expansion, including a relatively stiff annular portion displaced radially outward from said resonant cavity walls, clamping means having a, coefficient of linear expansion substantially smaller than that of the walls of said cavity disposed about said annular portion, annular fulcrums supported by said clamping means, at least one of said annular fulcrums being freely rotatable relative to said clamping means, said annular fulcrums engaging said annular portion at a predetermined position such that expansion of said walls parallel to the axis of said resonant cavity will be counterbalanced by lever multiplication of the expansion parallel to the axis of said cavity of said annular portion, and means for adjusting said clamping means to vary the spacing between said fulcrums.

2. The combination, with a walled resonant cavity, of means for compensating for the effects of thermal expansion, including a relatively stiff annular portion displaced radially outward from said resonant cavity walls, clamping means having a coefficient of linear expansion substantially smaller than that of the walls of said cavity disposed about said annular portion, annular fulcrums supported by said clamping means, at least one of said annular fulcrums being freely rotatable relative to said clamping means, said annular fulcrums engaging said annular portion at a predetermined position such that expansion of said walls parallel to the axis of said resonant cavity will be counterbalanced by lever multiplication of the expansion parallel to the axis of said cavity of said annular portion.

3. The combination, with a resonant cavity, of means for compensating for the effects of thermal expansion on frequency which include a relatively stiff annular portion displaced radially outward from said cavity, clamping means surrounding said annular portion having a thermal coefficient of expansion substantially different therefrom, annular fulcrum means supported by said clamping means and positioned so that expansion of that part of said annular portion disposed outside of said fulcrum means will act thereover to draw together that part of said annular portion disposed inside of said fulcrum means. 1

4. A structure comprising a first part formed as a hollow member with an open end; a second similarly formed part axially aligned with and spaced from said first part with the open ends of said two parts facing each other; a third part,

which is an extension of said first and second parts at the open ends thereof, formed as a hollow continuous member displaced radially outwardly from said first and second parts and extending around at least a portion of the periphery thereof; clamping means disposed about said third part and having a substantially difierent thermal coefiicient of expansion than said parts; and means mounted between said clamping means and said third part and responsive to changes in the dimensions of said third part to produce a compensating change of the dimensiontss of the space between said first and second par 5. The structure set forth in claim 4 wherein the walls thereof are thinner where said third part joins said first and second parts than the walls of the remainder of the structure.

6. A structure comprising a first part formed as a hollow member with an open end, a second similarly formed part having substantially the same peripheral configuration and wall thickness as said first part and being axially aligned with and spaced from said first part with the open ends of said two parts facing each other; a third part, which is an extension of said first and second parts at the open ends thereof, formed as a hollow continuous member displaced radially outwardly from said first and second parts and extending around at least a portion of the periphery thereof; clamping means disposed about said third part, said clamping means having a thermal coeff cient of expansion substantially different therefrom; fulcrum means supported by said clamping means and positioned against said third part so that expansion or contraction of said third part due to temperature changes act over said fulcrum means to produce a compensating change of the dimensions of the space between said first and second parts. 2

7. A structure comprising a first part formed as a hollow cylindrical member with an open and a closed end; a second similarly formed part having the same diameter and wall thickness as said first part and being axially aligned with and spaced from said first part with the closed ends of said parts remote from each other; a third part, which is an extension of said first and second parts at the open ends thereof, formed as a hollow portion with a U-shaped cross-sectional configuration, displaced radially outwardly from said first and second parts and extending at least around a portion of the circumference thereof; clamping means disposed about said third part having less thermal expansion than said third part for a given temperature change; fulcrum means supported by said clamping means and positioned against said third part so that expansion or contraction of said third part due to temperature changes act over said fulcrum means to produce a compensating change of the dimensions of the space between said first and second parts.

ALBERT D. EI-lRENFRIED.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,227,372 Webster et al Dec. 31, 1940 2,409,321 Stephan Oct. 15, 1946 

